Call responseCallback with the emitted status code

There is still situation where it is possible k6 will get status code
and then while it is reading the rest of the response an error will
occur - timeout reading the body, bad compression, etc. All of those
cases emit status code `0` and return status code `0` to the js script.

But responseCallback was getting the status code directly from the
response which meant that you can have an emitted metric with tag
`status` and `expected_response` where the two don't make sense as the
`status` is `0` but the `expected_response` was calculated based on a
different value.

While it might be more logical to return the status and the error in
this case intact this has been the case for nearly 3 years and given the
way the current js HTTP API is used this will likely just lead to very
confused users which got status `200`, checked on it and then their
response.body was cut halfway through.

lib/netext/httpext/request_test.go

@@ -230,6 +230,11 @@ func TestURL(t *testing.T) {
 
 func TestMakeRequestTimeout(t *testing.T) {
 	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Add("Content-Length", "100000")
+		w.WriteHeader(200)
+		if f, ok := w.(http.Flusher); ok {
+			f.Flush()
+		}
 		time.Sleep(100 * time.Millisecond)
 	}))
 	defer srv.Close()
@@ -246,14 +251,16 @@ func TestMakeRequestTimeout(t *testing.T) {
 		Transport: srv.Client().Transport,
 		Samples:   samples,
 		Logger:    logger,
+		BPool:     bpool.NewBufferPool(100),
 	}
 	ctx = lib.WithState(ctx, state)
 	req, _ := http.NewRequest("GET", srv.URL, nil)
 	preq := &ParsedHTTPRequest{
-		Req:     req,
-		URL:     &URL{u: req.URL, URL: srv.URL},
-		Body:    new(bytes.Buffer),
-		Timeout: 10 * time.Millisecond,
+		Req:              req,
+		URL:              &URL{u: req.URL, URL: srv.URL},
+		Body:             new(bytes.Buffer),
+		Timeout:          50 * time.Millisecond,
+		ResponseCallback: func(i int) bool { return i == 0 },
 	}
 
 	res, err := MakeRequest(ctx, preq)
@@ -262,14 +269,15 @@ func TestMakeRequestTimeout(t *testing.T) {
 	assert.Len(t, samples, 1)
 	sampleCont := <-samples
 	allSamples := sampleCont.GetSamples()
-	require.Len(t, allSamples, 8)
+	require.Len(t, allSamples, 9)
 	expTags := map[string]string{
-		"error":      "context deadline exceeded",
-		"error_code": "1000",
-		"status":     "0",
-		"method":     "GET",
-		"url":        srv.URL,
-		"name":       srv.URL,
+		"error":             "context deadline exceeded",
+		"error_code":        "1000",
+		"status":            "0",
+		"expected_response": "true", // we wait for status code 0
+		"method":            "GET",
+		"url":               srv.URL,
+		"name":              srv.URL,
 	}
 	for _, s := range allSamples {
 		assert.Equal(t, expTags, s.Tags.CloneTags())

lib/netext/httpext/transport.go

@@ -171,7 +171,7 @@ func (t *transport) measureAndEmitMetrics(unfReq *unfinishedRequest) *finishedRe
 	var failed float64
 	if t.responseCallback != nil {
 		var statusCode int
-		if unfReq.response != nil {
+		if unfReq.err == nil {
 			statusCode = unfReq.response.StatusCode
 		}
 		expected := t.responseCallback(statusCode)